Ureaplasma spp. colonize many healthy persons, yet they may also cause invasive diseases. Reasons they are commensals in some instances and produce systemic infections in others are unknown. There is increasing evidence that some Ureaplasma serovars may have a greater pathogenic potential than others. However, proof of this concept is incomplete. Prior attempts to study pathogenesis were hampered by imprecise typing methods, cross-reactions, lack of commercial reagents, and the fact that multiple serovars may be present simultaneously. Contradictory findings regarding differential pathogenicity of the 2 Ureaplasma species and individual serovars also suggests the possibility there may be virulence factors that were not detected using older, less discriminatory techniques. We hypothesize that differential pathogenicity of Ureaplasma spp. may be explained by analyzing clinical isolates and 14 serovars genotypically to identify genetic differences and possibly dissimilar expression of virulence factors. This research will examine Ureaplasma spp. from persons with invasive infections and compare them with others from persons without these conditions. Our Specific Aims are to: (1) Determine occurrence of Ureaplasma species and serovars in clinical isolates from a variety of different conditions and in commensal organisms using PCR;(2) Refine and further develop pulsed field gel electrophoresis (PFGE) and restriction fragment length polymorphism for use in determining genetic relatedness between Ureaplasma serovars and within serovars of pathogenic versus commensal isolates;(3) Compare the size and number of tandem amino acid repeats in multiple banded antigens of pathogenic versus commensal isolates a means to assess this characteristic as a predictor of disease;(4) Identify genes that encode potential virulence factors lgA1 protease and phospholipase A1, A2, and C activities using global transposon mutagenesis and comparative genomic analysis of all 14 serovars and selected clinical isolates to search for likely candidates for these genes, followed by cloning and expressing the genes to determine if their presence or expression correlates with pathogenic outcome. Identification of distinctive features of invasive strains may guide development of diagnostic tools and guide future studies to improve understanding of diseases affecting vulnerable populations including pregnant women and their infants, preventive strategies, and management.